This invention relates in general to television decoders and particularly to cable television decoders used for decoding television signals that include suppressed horizontal synchronizing pulses.
With the advent of subscription or "pay" television, various forms of encoding a television signal to restrict program viewing to subscribers having appropriate decoders has become very popular. One very common system randomly suppresses the synchronizing pulses and also inverts video information on a random field basis. Subscribers have suitable decoders which include a phase locked horizontal oscillator that, when unlocked, free runs at the horizontal frequency rate. A suppressed sync detector and restorer circuit samples the television signal when a sync pulse is expected, determining whether the sync pulse is suppressed, and if so, restores it. Such systems are well known in the art.
An occasional problem manifests itself in one such system when static video material is transmitted. The phase locked oscillator will often mistake the static video information for a synchronizing pulse and lock to it. Such a condition, called video lock out, will persist as long as the video remains static.
In the signal in that system, no suppression of sync is permitted during the vertical interval. It is during this portion of the signal, when unsuppressed sync pulses are present, that locking of the horizontal oscillator should occur. However, the phase lock loop may require a substantial number of fields before it is completely stabilized and properly restores all of the sync pulses that have been suppressed. The reasons for the oscillator failing to lock during the vertical interval are believed to be related to the disturbing effect that the vertical sync pulse and equalizing pulses have on the phase lock loop. Should the video information be negative going, i.e. have the same polarity as the sync pulses, the potentiality for video lock-out by the suppressed sync detector system mistaking static video for sync pulses is enhanced.
The problem is exacerbated because of the sync generating baseband decoders often used by cable operators for supplying coded television signals to their cable customers. A failure of a baseband decoder to properly sense suppressed sync denies service to all customers on the cable. Further, the cable operator usually tests equipment operation at remote locations by using color bars and other static test patterns and video lock out effectively precludes such testing.